I have taken a slightly different approach to the other suggestions and
would welcome all feedback on it.

The op's aim is to take a number of grades, average them and return a
grade not a number.

Firstly define a Name with a standard list of grades
eg, Grades ={"1a","1b","1c","2a","2b","2c","3a","3b","3c","4a ","4b","4c","5a","5b","5c"}

then the average grade from a range is
=INDEX(Grades,ROUND(AVERAGE(MATCH(A1:A10,Grades,0) ),0)) array entered -
control shift enter

change A1:A10 to reference a list of grades

what do you think?

Cheers RES